Cardiac pacemakers are medical devices, usually implantable, that provide electrical stimulation in the form of pacing pulses to selected chambers of the heart (e.g., the right atrium and/or the right ventricle). Pacemakers typically have a programmable electronic controller that causes the pacing pulses to be output in response to lapsed time intervals and sensed electrical activity (i.e., intrinsic heart beats). Implantable pacemakers sense intrinsic cardiac electrical activity by means of internal electrodes disposed near the chamber to be sensed, with the depolarization waves associated with contractions of the atria and ventricles referred to as P waves and R waves, respectively. In order to cause such a contraction in the absence of intrinsic activity, a pacing pulse (referred to as an A-pace or V-pace in the case of an atrium or ventricle, respectively) with energy above a certain pacing threshold is delivered to the chamber.
Most pacemakers are programmed to operate in a so-called demand mode (a.k.a., synchronous mode), where a pacing pulse is delivered to a heart chamber during a cardiac cycle only when no intrinsic beat by the chamber is detected. An escape interval is defined for each paced chamber, which is the minimum time interval in which a beat must be detected before a pace will be delivered. The ventricular escape interval thus defines the minimum rate at which the pacemaker will allow the heart to beat, sometimes referred to as the lower rate limit. If functioning properly, the pacemaker in this manner makes up for a heart's inability to pace itself at an appropriate rhythm.
In order for a pacemaker to control the heart rate in the manner described above, the paces delivered by the device must achieve “capture,” which refers to causing sufficient depolarization of the myocardium that a propagating wave of excitation and contraction result (i.e., a heart beat). A pacing pulse that does not capture the heart is thus an ineffective pulse. This not only wastes energy from the limited energy resources (battery) of pacemaker, but can have deleterious physiological effects as well, since a demand pacemaker that is not achieving capture is not performing its function in enforcing a minimum heart rate. A number of factors can determine whether a given pacing pulse will achieve capture, but the principal factor of concern here is the energy of the pulse, which is a function of the pulse's amplitude and duration or width. Programmable pacemakers enable the amplitude and pulse width of pacing pulses to be adjusted, along with other parameters. It is therefore desirable to perform a capture verification test at selected times in order to ascertain whether capture is being achieved by a pacemaker so that such parameters can be adjusted if needed.
A common technique used to determine if capture is present during a given cardiac cycle is to look for an “evoked response” immediately following a pacing pulse. The evoked response is the wave of depolarization that results from the pacing pulse and evidences that the paced chamber has responded appropriately and contracted. By detecting the evoked P-wave or evoked R-wave, the pacemaker is able to detect whether the pacing pulse (A-pulse or V-pulse) was effective in capturing the heart, that is, causing a contraction in the respective heart chamber. Capture verification can be performed in the clinical setting, with the clinician then adjusting pacing parameters so that the heart is reliably paced. It is desirable, however, for the pacemaker itself to be capable of verifying capture so that loss of capture can be detected when it occurs with pacing parameters then adjusted automatically, a function known as autocapture. An autocapture function provides the pacemaker with extended longevity, greater ease of use, and greater patient safety. In order for a pacemaker to detect whether an evoked P-wave or an evoked R-wave occurs immediately following an A-pulse or a V-pulse, a period of time, referred to as the atrial capture detection window or the ventricular capture detection window, respectively, starts after the generation of the pulse. Sensing channels are normally rendered refractory (i.e., insensitive) for a specified time period immediately following a pace in order to prevent the pacemaker from mistaking a pacing pulse or afterpotential for an intrinsic beat. This is done by the pacemaker controller ignoring sensed events during the refractory intervals, which are defined for both atrial and ventricular sensing channels and with respect to both atrial and ventricular pacing events. Furthermore, a separate period that overlaps the early part of a refractory interval is also defined, called a blanking interval during which the sense amplifiers are blocked from receiving input in order to prevent their saturation during a pacing pulse. If the same sensing channels are used for both sensing intrinsic activity and evoked responses, the capture detection window must therefore be defined as a period that supercedes the normal refractory period so that the sensing circuitry within the pacemaker becomes sensitive to an evoked P-wave or R-wave.
In certain devices, capture verification is performed by delivering a pacing pulse and attempting to sense an evoked response using the same electrode. Such a technique suffers from a number of problems, however. One is the induced polarization that builds up on an electrode after a pacing pulse which interferes with sensing by the electrode. Another is that an evoked response is a wave of depolarization that necessarily moves away from a pacing electrode responsible for the depolarization. Since such a wave of depolarization causes an electric field equivalent to a moving dipole disk, the potential resulting from that field is best sensed by an electrode that lies in the path of the wave. Also, if a backup pacing pulse is to be delivered, using the same pulse generator that produced the non-capturing pacing pulse for this purpose means that the output capacitor of the pulse generator must be recharged before another pace can be delivered, which takes some time.